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Molecular Interventions 2:141-144 (2002)
© 2002 American Society of Pharmacology and Experimental Therapeutics

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Emerging Roles for PKC Isoforms in Immune Cell Function

Thomas T. Su1, Beichu Guo2 and David J. Rawlings2,3

1 The Molecular Biology Institute, University of California, Los Angeles, CA 90095-1752, USA, and
2 Department of Immunology and
3 Department of Pediatrics, University of Washington School of Medicine, Seattle, WA 98195, USA

SUMMARY

Two publications by Mecklenbräuker et al. and Miyamoto et al. describe an essential role for protein kinase C{delta} (PKC{delta}) in the development of immune tolerance, and suggest that PKC{delta} also participates in mechanisms that prevent autoimmunity. Su, Guo, and Rawlings discuss these observations within the larger context of determining specific functions for individual isoforms of PKC in B and T lymphocytes, and the careful control these proteins exert in maintaining proper lymphocyte activation and tolerance.

The protein kinase C (PKC) family consists of thirteen members categorized as conventional, novel, atypical, or PKC-related isoforms, depending on whether diacylglycerol (DAG), calcium, or phosphatidylserine (PS) is required for their activation (1,2). Mice deficient in the conventional PKC isoform PKCß are immunodeficient, exhibiting a loss of peritoneal B-1a B cells and reduced T-cell–independent antibody responses (3). B cells from PKCß-deficient mice also fail to respond to BCR stimulation, suggesting that PKCß is a critical component of the BCR signaling machinery. Similarly, mature T lymphocytes in PKC{theta}-deficient mice exhibit defects in T cell antigen receptor- (TCR)-induced proliferation and reduced T-dependent responses (4). These results demonstrate the critical role for specific PKCs in B and T cell receptor signaling and function.

In contrast to the immunodeficiencies observed in PKCß and PKC{theta} mice, two recent reports demonstrate that mice deficient in the novel PKC isoform PKC{delta} exhibit B cell hyperactivation, which leads to autoimmunity (5,6). Whereas early B cell development in the bone marrow of PKC{delta}-deficient mice is normal, exaggerated B cell expansion occurs in the spleen and other peripheral organs, resulting in splenomegaly and lymphadenopathy. The presence of excessive serum autoantibodies specific for DNA and nuclear proteins led to immune-complex deposition and glomerulonephritis in PKC{delta}–/– mice. Furthermore, transplantation of PKC{theta}-deficient B cells into normal animals (i.e., adoptive transfer) indicated that this hyperproliferative phenotype was B cell specific (5), underscoring the essential role for PKC{delta} in negative regulation of B lymphocyte signaling.

The autoimmune phenotype of PKC{delta}–/– mice was associated with a loss of peripheral tolerance. For example, the B cells of mice that carry a BCR transgene that specifically recognizes hen egg lysozyme (IgHEL) will undergo negative selection (i.e., activation-induced cell death) in the periphery of transgenic mice that have been bred with transgenic mice expressing soluble hen egg lysozyme (sHEL) (7,8). However, negative selection observed in doubly transgenic IgHEL–sHEL mice was abrogated in the absence of PKC{delta}, suggesting a critical role for PKC{delta} in regulating peripheral B cell tolerance (6).

The mechanisms responsible for the breakdown of tolerance in PKC{delta}–/– B cells remain unclear. Peripheral B lymphocytes develop in a step-wise fashion from immature transitional 1 (T1) to transitional 2 (T2) to naïve, follicular mature B cells (9). T1 and T2 cells reside in distinct splenic microenvironments and exhibit differential responsiveness to BCR engagement whereby T1 B-cells might be the target for peripheral negative selection, and T2 B-cells might be the target for BCR-dependent positive selection (10). Evaluation of the signaling functions of PKC{delta} specifically within T1 vs. T2 peripheral B cell populations might therefore provide important insight into how PKC{delta} functions to regulate peripheral B cell tolerance.

Notably, the data from Miyamoto et al. suggest that PKC{delta} may regulate B cell activation by inhibiting interleukin (IL)-6 production. IL-6 expression and secretion are increased in PKC{delta}–/– splenic B cells (5). This is consistent with the exaggerated B cell expansion and plasmacytosis observed in IL-6 transgenic mice (11). IL-6 expression is dependent on the transcriptional activator NF-IL6, or the CCAAT/enhancer-binding protein (C/EBPß). PKCs can phosphorylate NF-IL6 in vitro on inhibitory residue Ser240, leading to a marked decrease of NF-IL6 DNA-binding activity (12). Lipopolysaccahride- (LPS)-induced NF-IL6 DNA binding activity was greatly increased in PKC{gamma}–/– B cells (5), suggesting that PKC{delta} may regulate B cell tolerance, in part, by inhibiting IL-6 production through site-specific phosphorylation of NF-IL6.

Interestingly, PKCß appears to possess a similar ability to phosphorylate critical inhibitory residues on Bruton’s tyrosine kinase (Btk). PKCß is activated upon BCR stimulation, in response to increased concentrations of DAG and Ca2+. Sustained generation of these two second messengers is mediated by Bruton’s tyrosine kinase (Btk) through the activation of PLC-{gamma} isoforms (Figure 1Go) (13). Although PKCß has a definite role in mediating the positive effects downstream of Btk, PKCß (and possibly other conventional PKCs) can also inhibit Btk through a negative feedback loop (14). PKCß specifically phosphorylates Btk on Ser180 within the Tec homology (TH) region, leading to the inhibition of Btk membrane translocation and activation (14). In addition, the atypical PKC isoform PKCµ (or PKD) also negatively regulates BCR signaling through the phosphorylation-dependent inhibition of the tyrosine kinase Syk (15). Therefore, the ability to negatively regulate lymphocyte signaling by site-specific phosphorylation at inhibitory residues may be a common characteristic of many PKC family members (Figure 1Go).



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  		Figure 1. Negative roles for PKC isoforms in B lymphocyte signaling. B cell receptor (BCR) engagement leads to a cascade of signaling events, eventually leading to the activation of the B cell. Specific PKC isoforms that are activated by the BCR can inhibit B cell signaling. Although PKCß and PKCµ function in negative feedback loops to inhibit the upstream activators Btk (itself activated bt the tyrosine kinase Lyn) and Syk, respectively, PKC{delta} may function to inhibit the NF-IL6 transcription factor. BLNK, B cell linker protein; Syk, a 72-kDa protein tyrosine kinase highly expressed in B cells; Btk, Bruton’s tyrosine kinase; DAG, diacylglycerol; NF-IL6, nuclear factor for interleukin-6 expression.

 
In addition to the negative effects mediated by PKC{delta} on lymphocyte activation, an earlier study using chemical inhibitors suggested that PKC{delta} functions in a positive role in BCR dependent NF–{kappa}B activation (16). In contrast to these findings, the reports by Miyamoto et al. and Mecklenbräuker et al. demonstrate that NF–{kappa}B activation in B cells is completely intact in PKC{delta}-deficient mice (5,6). These differences in results emphasize that caution should be maintained when using inhibitors with partially overlapping specificity to assess the physiologically relevant function of proteins.

In contrast to the intact NF–{kappa}B function in PKC{delta}–/– mice, recent studies demonstrate that PKCß is the major PKC isoform required for BCR-dependent NF–{kappa}B activity. Splenic B cells from PKCß-deficient mice exhibit severe defects in survival, Bcl-xL induction, I{kappa}B degradation, and I{kappa}B kinase (IKK) activity in response to BCR engagement (17). The mechanism of NF–{kappa}B regulation by PKCß appears to be mediated through PKCß-dependent recruitment of IKK into the membrane lipid rafts that are associated with the BCR signaling complex. In contrast, CD40-dependent NF–{kappa}B activity is fully intact in PKCß–/– B cells, suggesting that this role for PKCß is specific to BCR signaling. Similar to the role for PKCß in B cells, TCR-dependent activation of NF–{kappa}B is abrogated in splenic T cells from PKC{theta}–/– mice (4), suggesting an analogous function for PKC{theta} in regulating TCR-dependent recruitment of IKK into lipid rafts and NF–{kappa}B activation. Together, these findings suggest that PKCß and PKC{theta} are responsible for cell-specific and receptor-specific activation of the NF–{kappa}B signaling pathway in B and T lymphocytes, respectively (Figure 2Go).



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  		Figure 2. Positive roles for PKC isoforms in lymphocyte signaling. Engagement of the BCR (A) or the T cell antigen receptor (TCR) (B) activates a set of similarly acting signaling molecules. Specific Src (Lyn and Lck), Syk-Zap70, and Tec (Btk and Itk/Rlk) family kinases are activated. These events mediate increased activity of PLC-{gamma}, leading to the generation of the second messengers DAG and calcium, which are required for the activation of most PKCs. Both PKCß and PKC{theta}, respectively, function in controlling BCR and TCR dependent NF–{kappa}B activation. SLP-76, src homology (SH)2 domain–containing leukocyte protein of 76-kD.

 
Implicit in the identification of specific roles for individual PKC isoforms in positive or negative regulation of lymphocyte signaling is that PKCs may represent useful potential drug targets for various immune disorders. For example, the autoimmune disease observed in PKC{delta}–/– mice suggests that negative regulatory functions by PKCs are crucial for maintaining proper immunological tolerance. It remains to be seen whether PKC{delta} function is reduced in certain autoimmune states, and whether specific activators of PKC{delta} can be developed to treat such conditions. Furthermore, because PKC{delta} and PKCß seem to have negative and positive roles, respectively, in B lymphocytes, it will be interesting to test whether concomitant PKCß-deficiency might abrogate the autoimmunity observed in PKC{delta}–/– mice.

Chemical activators such as TPA (12-O-tetradecanoylphorbol-13-acetate) are not specific for individual PKCs, nor are PKC isoform-specific activators available (18,19). In contrast, a number of inhibitors specific for individual PKCs do exist including antisense oligonucleotides and chemical inhibitors of catalytic activity (18). The positive roles for PKCß and PKC{theta} in lymphocyte function suggest that highly specific inhibitors of these PKC isoforms may have great potential in treating B-cell– or T-cell–specific disorders. Although no PKC{theta}-specific chemical inhibitors exist, several PKCß-specific inhibitors are currently available, including the macrocyclic bis (indolyl) maleimides LY-333531, LY-379196 and LY-317615 (20). These compounds are well tolerated systemically and can be administered orally (21). The availability, safety, and efficacy of PKCß-specific oral inhibitors suggest that they may also be useful in the treatment of certain B-cell immune disorders. For example, crossing Btk-deficient mice with lupus-prone (NZB x NZW)F1 mice abrogates the systemic lupus erythematosus (SLE)-like symptoms in these animals (22). Similar experiments involving the breeding of PKCß–/– mice to (NZB/W)F1, to PKC{delta}–/–, or to other autoimmune-prone mice will be required to validate PKCß as a drug target for B-cell–dependent autoimmune diseases. In addition, a clinically refractory subset of non-Hodgkin’s diffuse large B cell lymphomas (DLBCLs) exhibit elevated PKCß expression (23). Recent studies in our laboratory demonstrate efficacy for PKCß-inhibitors in specifically blocking the survival of PKCß-expressing DLBCL tumor lines in vitro (17).

Further studies will be needed to confirm the efficacy of PKCß-specific inhibitors in treating B cell malignancies and autoimmune disease in vivo. The future development of PKC{theta}-specific inhibitors and PKC{delta}-specific activators may similarly lead to effective treatments for T cell lymphomas and various autoimmune conditions. Therefore, understanding the positive and negative regulation of PKC function in the immune system may lead to effective treatments for human disease in the near future.


David J. Rawlings, MD, is the Head of the Section of Pediatric Immunology/Rheumatology at the University of Washington and Children’s Hospital Medical Center, Seattle; and Associate Professor of Pediatrics and Immunology, in the School of Medicine at the University of Washington. Address comments to DJW. E-mail: drawling{at}u.washington.edu


Thomas T. Su is an MD-PhD student in the School of Medicine and the Molecular Biology Institute, University of California, Los Angeles, and is a member of Rawlings laboratory.


Beichu Guo, MS, is a PhD student in the Department of Immunology at the University of Washington, and is a member of Rawlings laboratory.

References

  1. Newton, A.C. Regulation of protein kinase C. Curr. Opin. Cell Biol. 9, 161–167 (1997).[CrossRef][Medline]
  2. da Rocha, A.B., Mans, D.R., Regner, A., and Schwartsmann, G. Targeting protein kinase C: New therapeutic opportunities against high-grade malignant gliomas? Oncologist 7, 17–33 (2002).[Abstract/Free Full Text]
  3. Leitges, M., Schmedt, C., Guinamard, R., Davoust, J., Schaal, S., Stabel, S., and Tarakhovsky, A. Immunodeficiency in protein kinase Cß-deficient mice. Science 273, 788–791 (1996).[Abstract]
  4. Sun, Z., Arendt. C.W., Ellmeier, W. et al. PKC-{theta} is required for TCR-induced NF–{kappa}B activation in mature but not immature T lymphocytes. Nature 404, 402–407 (2000).[CrossRef][Medline]
  5. Miyamoto, A., Nakayama, K., Imaki, H. et al. Increased proliferation of B cells and auto-immunity in mice lacking protein kinase C{delta}. Nature 416, 865–869 (2002).[CrossRef][Medline]
  6. Mecklenbräuker, I., Saijo, K., Zheng, N.Y., Leitges, M., and Tarakhovsky, A. Protein kinase C{delta} controls self-antigen-induced B-cell tolerance. Nature 416, 860–865 (2002).[CrossRef][Medline]
  7. Goodnow, C.C., Crosbie, J., Adelstein, S. et al. Altered immunoglobulin expression and functional silencing of self-reactive B lymphocytes in transgenic mice. Nature 334, 676–682 (1988).[CrossRef][Medline]
  8. Hartley, S.B., Crosbie, J., Brink, R., Kantor, A.B., Basten, A., and Goodnow, C.C. Elimination from peripheral lymphoid tissues of self-reactive B lymphocytes recognizing membrane-bound antigens. Nature 353, 765–769 (1991).[CrossRef][Medline]
  9. Loder, F., Mutschler, B., Ray, R.J., Paige, C.J., Sideras, P., Torres, R., Lamers, M.C., and Carsetti, R. B cell development in the spleen takes place in discrete steps and is determined by the quality of B cell receptor-derived signals. J. Exp. Med. 190, 75–89 (1999).[Abstract/Free Full Text]
  10. Su, T.T. and Rawlings, D.J. Transitional B lymphocyte subsets operate as distinct checkpoints in murine splenic B cell development. J. Immunol. 168, 2101–2110 (2002).[Abstract/Free Full Text]
  11. Suematsu, S., Matsuda, T., Aozasa, K. et al. IgG1 plasmacytosis in interleukin-6 transgenic mice. Proc. Natl. Acad. Sci. U.S.A. 86, 7547–7551 (1989).[Abstract/Free Full Text]
  12. Trautwein, C., van der Geer, P., Karin, M., Hunter, T., and Chojkier, M. Protein kinase A and C site-specific phosphorylations of LAP (NF–IL6) modulate its binding affinity to DNA recognition elements. J. Clin. Invest. 93, 2554–2561 (1994).
  13. Fluckiger, A.C., Li, Z., Kato, R.M. et al. Btk/Tec kinases regulate sustained increases in intracellular Ca2+ following B-cell receptor activation. EMBO J. 17, 1973–1985 (1998).[CrossRef][Medline]
  14. Kang, S.W., Wahl, M.I., Chu, J. et al. PKCß modulates antigen receptor signaling via regulation of Btk membrane localization. EMBO J. 20, 5692–5702 (2001).[CrossRef][Medline]
  15. Sidorenko, S.P., Law, C.L., Klaus, S.J., Chandran, K.A., Takata, M., Kurosaki, T., and Clark, E.A. Protein kinase C mu (PKCµ) associates with the B cell antigen receptor complex and regulates lymphocyte signaling. Immunity 5, 353–363 (1996).[CrossRef][Medline]
  16. Krappmann, D., Patke, A., Heissmeyer, V., and Scheidereit, C. B-cell receptor- and phorbol ester-induced NF–{kappa}B and c-Jun N-terminal kinase activation in B cells requires novel protein kinase Cs. Mol. Cell. Biol. 21, 6640–6650 (2001).[Abstract/Free Full Text]
  17. Su, T.T., Guo, B., and Rawlings, D.J. Unpublished results.
  18. Carter, C.A. Protein kinase C as a drug target: Implications for drug or diet prevention and treatment of cancer. Curr. Drug Targets 1, 163–183 (2000).[CrossRef][Medline]
  19. Barry, O.P. and Kazanietz, M.G. Protein kinase C isozymes, novel phorbol ester receptors and cancer chemotherapy. Curr. Pharm. Des. 7, 1725–1744 (2001).[CrossRef][Medline]
  20. Goekjian, P.G. and Jirousek, M.R. Protein kinase C inhibitors as novel anticancer drugs. Expert Opin. Investig. Drugs 10, 2117–2140 (2001).[CrossRef][Medline]
  21. Frank, R.N. Potential new medical therapies for diabetic retinopathy: Protein kinase C inhibitors. Am. J. Ophthalmol. 133, 693–698 (2002).[CrossRef][Medline]
  22. Steinberg, B.J., Smathers, P.A., Frederiksen, K., and Steinberg, A.D. Ability of the xid gene to prevent autoimmunity in (NZB x NZW)F1 mice during the course of their natural history, after polyclonal stimulation, or following immunization with DNA. J. Clin. Invest. 70, 587–597 (1982).
  23. Shipp, M.A., Ross, K.N., and Tamayo, P. et al. Diffuse large B-cell lymphoma outcome prediction by gene expression profiling and supervised machine learning. Nat. Med. 8, 68–74 (2002).[CrossRef][Medline]



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